1. Field of the Invention
The invention relates to a novel process for preparing alkyl oligoglycosides which have C.sub.8 to C.sub.20 alkyl chains by acid-catalyzed transglycosidation of C.sub.2 to C.sub.6 alkyl glycosides using C.sub.8 to C.sub.20 fatty alcohols.
2. Discussion of the Background
Alkyl oligoglycosides having C.sub.8 to C.sub.20 alkyl groups can be prepared in whole or in part from renewable raw materials. Therefore, and also because of their very good biodegradability, they are becoming increasingly important. In addition to their surfactant properties of interest, the products have the advantage that their polarity can be set exactly via the length of the alkyl chain and via the degree of glycosidation. By this means the alkyl oligoglycosides can be specifically directed to their field of application.
In the two-stage alkyl oligoglycoside preparation, a glycosidation is first carried out. Alkyl glycosides having short-chain alkyl groups are prepared from saccharides and C.sub.2 to C.sub.6 alcohols in this stage. These products are then converted in the second stage by transglycosidation using C.sub.8 to C.sub.20 alcohols into the desired alkyl oligoglycosides having surfactant properties.
This preparation route has long been known. Recent applications in this sector frequently concern the preparation of products improved in color. Reducing agents are sometimes added or special equipment is used. However, the transglycosidation is usually performed in an externally heated stirred tank or in a heated stirred tank cascade.
According to EP-A-0 514 628, the transglycosidation can also be carried out in an evaporator. According to EP-A-0 514 627, the transglycosidation is carried out in a reaction column. A molar ratio of alkyl glycoside having a short alkyl radical to long-chain alcohol of 1:2 to 1:15 is set in this case and alkyl oligoglycosides are obtained having degrees of glycosidation of preferably 1.2 to 3.
WO 93/101 33 describes a two-stage process for preparing alkyl oligoglycosides which can be carried out continuously or discontinuously and in which a glucose syrup having a content of monomeric glucose of 90 to 100% is used. The process described here is highly complex overall. It requires a secondary reaction both in the glycosidation and also in the transglycosidation.
In the transglycosidation here, the fatty alcohol which has been heated to the reaction temperature is introduced into a stirred tank having jacket heating. The short-chain alkyl glycoside is added. During the transglycosidation the mixture is passed into an evaporator at a low vacuum of about 100 mbar.
In the said processes, the heat for the reaction temperature is introduced externally via the reactor jacket.
However, this procedure has the disadvantage that the wall temperature is generally at least 5.degree. to 10.degree. C. above the reaction temperature. This can cause sensitive products to be thermally damaged and discolored at the reactor wall. Encrustations and blockages in the equipment and piping can therefore occur.
The object of the present invention was therefore to avoid overheating, in particular due to elevated jacket temperatures, in the transglycosidation.